Robust electronic phase transition against cation disorder in high-entropy pyrochlore iridates

Abstract

High-entropy pyrochlore iridates A2Ir2O7 with multiple trivalent A cations were synthesized. The parent ternary A2Ir2O7 displays a variety of electronic phases depending on the size of A cations; Pr2Ir2O7 with a large A cation shows semimetallic behavior down to low temperatures, whereas A2Ir2O7 with smaller A cations, such as Nd2Ir2O7 and Eu2Ir2O7, displays a (semi)metal to magnetic insulator transition as a function of temperature. By further reducing the A cation size, smaller than Y3+, A2Ir2O7 becomes a Mott insulator, and long-range magnetic order takes place below room temperature. The metal–insulator transition and magnetic ordering turned out to be robust against strong disorder induced by the mixing of more than five A-cations in the high-entropy A2Ir2O7. The transition temperatures were found to scale with the average ionic radius of multiple A-cations. In contrast, high-entropy A2Ir2O7 including Bi3+ exhibits metallic behavior down to 2 K, which is likely associated with the presence of oxygen vacancies as in the parent Bi2Ir2O7. Although these indicate that the overall electronic structure of A2Ir2O7 remains intact in the presence of high-entropy configuration at the A-site, the transport properties suggest that fine details of the band structure may be modulated by local distortion. Strong disorder at the A-site of complex oxides may be exploited as a tool to control electronic properties.